The instantaneous electrical power (watts) supplied to a load is measured by multiplying the instantaneous load voltage times the instantaneous load current. The energy consumed by a load (watt hours) is measured by integrating the instantaneous power over time. Alternating current power has been measured in the past by multiplying the peak voltage and peak current together with a power factor, but this requires circuitry for measuring the peak values of the voltage and current components, and further circuitry to determine the difference in the phase angles between the voltage and current components. Other devices sampled the current only when the voltage component was at its peak value, or vice versa. While this eliminates the need to determine the phase angle, it has experienced limitations with respect to the sampling rate or frequency of operation.
Still other prior art devices use pulse width modulation techniques to multiply instantaneous voltage and current signals, and thereby obtain a product signal representing the instantaneous power. For example, a load current signal is supplied to a duty cycle modulator which generates a train of pulses of constant amplitude and constant frequency, but with a duration, or width, that is modulated in response to the load current signal. This train of pulses typically gates a switching element, with the duty cycle, or "on" time, of the switching element varying in response to the width of the pulses in the pulse train. Such prior art devices may operate under ideal or laboratory conditions, but suffer degradations in accuracy in the presence of harmonics in the load current and load voltage signals, and have restrictions on their operational ranges due to the effect of variations in environmental temperature, power factor and load. Specifically, pulse width modulation circuits, and other electronic circuits for measuring electric energy, typically require accurate clock and time base circuits to ensure the very high degree of accuracy required by electric utilities. Unfortunately, these circuits are complex and expensive, and like most semiconductor devices, sensitive to variations in temperature, circuit bias conditions, and non-ideal signals. Other prior art electronic circuits for measuring the electric energy used by a load suffer from a narrow bandwidth of operation. This means they are unable to measure power over a wide frequency range and still remain accurate in the presence of harmonics or other variations in the load voltage and load current signals.
Accordingly, it is an object of the present invention to provide a method and apparatus for electronically measuring the amount of electric power used by a load.
It is a further object of the present invention to provide a method and apparatus for accurately measuring the amount of electric power used by a load over a broad bandwidth, and provide an accurate measure even in the presence of harmonics in the load current and load voltage signals.
It is a still further object of the present invention to provide a method and apparatus for electronically measuring the amount of electric power used by a load using digital techniques when the voltage and current applied to the load are analog signals.
It is a still further object of the present invention to provide a method and apparatus for electronically measuring electricity.
These and other objects of the present invention are accomplished with the method and apparatus of the present invention. In one embodiment for measuring power used by a load, the apparatus includes means for generating a rectified voltage signal proportional to the load voltage. A frequency modulated pulse train is generated in response to the load voltage signal, with the frequency of the pulses being proportional to the instantaneous amplitude of the rectified load voltage. A load current signal is generated that is proportional to the load current. A logic circuit gates the load current signal during each pulse of the frequency modulated pulse train to generate a signal representing the product of the instantaneous load voltage and the instantaneous load current. This signal from the logic means may be integrated over time to generate a signal representing the amount of electric energy used by the load. A method for practicing this invention is also disclosed.